Particle board is a composite material which is typically manufactured from various kinds of wood particles, such as wood chips, sawmill shavings or saw dust. It can be used especially for the manufacture of various types of furniture, such as cabinets, kitchens, bath furniture and so forth. Generally, particle board (which is sometimes referred to as “chipboard”) is usually produced by mixing wood particles and a binder composition, e.g. a thermo-curable resin, subsequently forming the resulting mixture into a sheet and compressing said sheet under elevated temperatures. Most commonly, urea formaldehyde (UF), melamine urea formaldehyde (MUF) and phenol formaldehyde have been used as binder systems in particle board. Moreover, a variety of additives can be used, including waxes, dyes and release agents, which supplement the particle board with further desired properties. In order to improve the visual appearance and durability of particle board, veneer can be applied onto its visible surfaces.
Particle board has a variety of advantages over conventional wood products and plywood since it is cheaper, and generally denser and more uniform. As a consequence, in many applications particle board has replaced these conventional materials, predominantly for cost reasons.
However, one major disadvantage of particle board in certain applications is that it undergoes significant swelling upon exposure to moisture over a prolonged period of time. This is particularly based on the fact that, particularly urea formaldehyde particle board, is considerably hygroscopic. Exposure to humidity or direct contact with water usually leads to swelling of the material. As a consequence, appearance and strength of the particle board are significantly impaired. Due to this sensitivity to moisture, the application of particle board in humid environments can be disadvantageously restricted.
Resistance to moisture and swelling are also important considerations for other types of wood boards, for example plywood, although the structure of plywood generally renders it less susceptible to moisture and swelling that particle board.
A wood board must, of course, satisfy a combination of requirements including satisfactory internal bond strength, modulus of elasticity, bending strength and dimensional stability. Resistance against decay, for example due to mould and/or biodegradation is also important.
It has previously been attempted to manufacture urea formaldehyde particle board having reduced swelling when exposed to moisture, e.g. by using higher amounts of binder. However, the use of significantly higher amounts of binder negatively affects the particle board's overall properties as well as cost efficiency and the cured UF binders still tend to be hydroscopic.
Moreover, it has been suggested to reduce swelling in UF/MUF particle board by applying heat to the finished product, whereby the particle board generally becomes less hygroscopic and hot pressing stress is relieved. However, even though such techniques may basically lead to reduced swelling of the particle board, the extensive application of heat also leads to degradation of the cured UF/MUF binders whereby the structural integrity of the product is negatively influenced.
One specific technical problem underlying the present invention is thus to overcome the above negative effects and to provide particle board with reduced swelling in the thickness direction and a process for producing such particle board.